This invention relates to an indicator device, more particularly a rapid response device for the detection of carbon dioxide in a gas mixture. The invention is also concerned with a method for determining the correct placement of an endotracheal catheter in the trachea of a patient, particularly during apnea, by use of such indicator device.
Devices for the detection of carbon dioxide which rely in part upon the change in color of certain chemical compounds according to the pH of their environment are known in the art. Such chemical indicators change color in solution when the pH of the solution changes.
Numerous examples of chemical indicators which are pH-sensitive, and thereby useful in carbon dioxide indicator systems, have been disclosed in the prior art.
U.S. Pat. No. 2,890,177 discloses a liquid chemical indicator for detecting the presence of carbon dioxide in respiratory gases comprising an aqueous solution of an alkali metal oxalate and a pH sensitive dye which changes color at a pH in the range of 6.6 to 5.8.
U.S. Pat. No. 3,068,073 discloses a method of determining carbon dioxide in a gas which comprises passing the gas to be tested through a solid reagent comprising activated alumina carrying thymol blue and, optionally, a base.
U.S. Pat. No. 3,114,610 discloses a continuous sampling gas analyzer comprising a pH sensitive dye suspended in a gel substance, a semi-permeable membrane which separates the gas to be tested from the dye but allows gas to pass therebetween for analysis, a light source for illuminating the dye and a detector for analyzing the light transmitted through said dye. For carbon dioxide determination the dyes disclosed are Methyl Red and Bromcresol Green.
U.S. Pat. No. 3,754,867 discloses a gas analyzer which uses a light source to transmit light through a multi-layered sensor unit and a detector to receive and analyze the color change of a pH-sensitive indicator in one of said layers, said color change being a function of the concentration of carbon dioxide in the gas being measured. Examples of acid base indicators disclosed are phenol red, brilliant yellow, meta-cresol purple, cresol red, neutral red, m-nitrophenol and m-dinitrobenzoylene urea.
The various devices and compositions disclosed in the above-mentioned prior art references provide means for detecting or indicating the presence of carbon dioxide under certain circumstances. However, none of the said references directly addresses the problem of determining accurately and rapidly the correct positioning of an endotracheal catheter in the trachea of an apneic patient.
Introduction of a catheter in the trachea of a human may be required for a number of reasons. For example, in a hospital, an endotracheal catheter, also known as an intratracheal catheter, may be used for general anesthesia; in the field, a doctor or para-medic may use an endotracheal catheter to resuscitate an apneic patient. In both of these instances, and others, it is critical that the catheter be properly placed in the trachea and not, for example, in the esophagus. If the catheter is improperly placed and the error is not discovered within a very short time, of the order of 5 to 20 seconds, the patient may begin to suffer irreparable harm or even death.
In view of the criticality of the timing when an endotracheal catheter is improperly placed in an apneic patient, there is clearly a need for a simple device which will rapidly and reliably give an indication of improper (or proper) placement. (See P. K. Birmingham et al, "Esophageal Intubation", ANESTH ANALG, 1986, 65, 886-91).
A device for simplifying, expediting and making safe the technique of introduction of an intratracheal catheter is disclosed in U.S. Pat. No. 2,638,096. This device uses a perforate whistle which is adapted to sound an audible signal, and thus give evidence of even feeble breathing, when the catheter is properly placed in the trachea. However, the device is designed for the administration of general anesthesia to a patient with spontaneous respiration and the absence of an audible signal, for example when treating an apneic patient, does not necessarily mean that the catheter has been improperly placed.
It is known that the concentration of carbon dioxide in the atmosphere is normally about 0.03%, whereas the concentration of carbon dioxide in the gas exhaled by a human being is normally 4.5 to 5.0%. The normal amount of carbon dioxide in the esophagus of a human being is negligible. Accordingly it is evident that a rapid, accurate determination of the presence (or absence) of carbon dioxide in the gas exhaled by a human being will provide a clear indication as to whether the tube carrying said gas is properly placed in the trachea or improperly placed in the esophagus.
An apparatus for indicating the presence and relative amounts of carbon dioxide in gases breathed by patients is disclosed in U.S. Pat. No. 2,136,236. However, this apparatus utilizes a liquid solution for detection and it is concerned with the determination of gases in a closed breathing circuit, such as an oxygen tent.
It has now been found that the correct placement of an endotracheal catheter can be determined simply and rapidly by attaching to the distal end of said catheter a device embodying a rapid response carbon dioxide indicator.